Economic considerations make it desirable to extract, through each oil or gas well drilled, as much of the desired product as possible. As the size of the wellbore is always extremely small relative to the area of the production formation, the migration of the products to the well is a major concern in oil and gas production. Problems can arise if the product is impeded from easily migrating to the extraction well due to the restrictive physical properties of the matrix in which it lies. With gas and oil wells one way to solve this problem is to fracture or create cracks in the rock formation around the bottom of the well, so the desired product can move to the well more freely.
A common method of creating cracks in rock formations is called foam fracturing. Gelled fluid (usually water) is mixed with a particulate proppant (usually coarse sand). The proppant material becomes suspended in the gelled fluid. A foaming agent and delayed action gel breaker are added. Then, the slurry is pumped down the well at a very high pressure. However, just before it goes down the well a gas (usually nitrogen) is injected into the proppant/fluid slurry in a ratio of about three parts gas to one part fluid. This mixture creates a foam. At the production formation level there are a series of holes in the well casing through which the foam can move into the rock formation. When the pressure becomes high enough the formation starts to crack. The pressure is maintained as the crack enlarges and the proppant bearing foamed gel is injected into the fracture. Then, after a short time, the gel breaker causes the liquid to regain its original viscosity. A valve is opened at the wellhead and pressure from the gas injected into the fracture blows the fluid back out to the surface, leaving the proppant caught in the crack to hold it open. The presence of the fracture allows the product easier access to the well for removal to the surface.
As the primary function of the fluid in the injected slurry is simply to act as a carrier for the proppant and as the presence of water can often cause serious problems in the production zone, it is often desirable to fracture using a high proppant/fluid ratio slurry. Although this requirement most often arises when water is being used as the carrier medium, it is often advantageous to have high proppant concentrations when using other liquids as well.
If it were not for the physical limitations of the high pressure pumps used, increasing proppant concentrations would not be a problem. However, even the most modern and efficient pumps will not pump proppant/fluid slurry at concentrations much higher than ten pounds of proppant per U.S. gallon of liquid, at the pressures required for fracturing. At concentrations above this critical ratio the pumps tend to simply plug up with proppant and stop. These requirements and problems have led to the development of a proppant concentrator which is placed in high pressure frac lines between the pumps and the wellhead. By removing fluid at that point the concentrator allows the slurry to pass through the pumps with enough fluid to avoid "sand-offs" while creating a high proppant-ratio slurry before injection into the production formation.
Sand concentrations for use in fracturing have previously been proposed. An example is in a preprint by the Petroleum Society of CIM. Paper No. 79-30-39 entitled "The Sand Intensifier--Its development and application to fracturing" for a paper presented May 8, 11, 1979. The apparatus described was a sand concentrator and not a combined concentrator and foamer. There was a cylindrical casing containing an impeller which is rotated with consequent problems of high abrasive wear, shaft sealing, and the danger of stalling due to piling up of sand. There is no straight through axial flow of both inlet and outlets. The outlets are in the sides of the cylinder and a central diaphragm seals the slurry from the clean water section.